1. Field of the Invention
The invention relates to a method for the controlled withdrawal movement of a machining electrode in an erosion device, and to a corresponding control device for performing this method.
2. Description of Related Art
In DE-PS-35 25 683 a method is disclosed wherein, when a short-circuit occurs, the machining electrode first moves back along the previously traveled machining path by a predefined, short section inside the already eroded path (first way of withdrawal; from hereon also called the "default" way of withdrawal). If, at the end of this first type of withdrawal, the short-circuit has not been corrected yet, the electrode is moved further away from the short-circuit on a second, straight path. This second path is described by a withdrawal sector whose direction and length have been chosen so that the machining electrode is removed from the short-circuit point as quickly as possible (second type of withdrawal).
A method employing only the "default" way of withdrawal is known, for example, from DE-OS 37 05 475 for an electrical arc device. This publication furthermore describes a "point" way of withdrawal in which the electrode is moved towards a point when a short-circuit occurs. This point is hereby adapted to the current machining site, which means, for example, that it follows it into the depth of the tool when the erosion is performed into it.
DE-PS 38 17 302 describes a further development of the above mentioned process in which the electrode is moved first along the first, then along the second, and then along a third way of withdrawal. The third way of withdrawal follows the second type of withdrawal at the time at which the tip of the withdrawal vector is reached. The third way of withdrawal then takes place parallel to the eroded machining path, in an opposite direction. The withdrawal vector for the second way of withdrawal is also defined so as to have a fixed length and direction (from hereon also called the "fixed vector" way of withdrawal.)
The disadvantage of the known withdrawal strategies is that the machining electrodes are removed either only slowly from the short-circuit point in the initial phase, in particular, they are not moved fast enough from the already eroded machining path (for example, with the "default" way of withdrawal), or there is a risk that the electrodes will damage smaller edges created during the withdrawal movement (for example, with the "fixed vector" and "point" types of withdrawal. These edges are created in such a way, for example, that in the case of a short-circuit the electrode erodes the work piece in such a manner that a curvature corresponding to the electrode curvature is created in the work piece. If the machining electrode is removed only slowly from the short-circuit point, the erosion gap also does not significantly increase at the beginning of the withdrawal movement. The erosion particles causing the short-circuit at this point therefore cannot be flushed quickly enough from the narrow erosion gap, so that the short-circuit continues.
In addition, most of these withdrawal strategies show little consideration of the actual progression of the machining path, resulting in undesired collisions of the machining electrode with the processed work piece. This is the case particularly if the machining path has small, localized directional changes that are not detected by any withdrawal vectors.